Looking for a recommendation

I am 16 stones and 5ft 5, my partner is 15 stones at 5ft 1.
We are looking for a firmer mattress than what we have now
The current mattress has dipped in a few places and doesn't give us enough support.
I have slipped disc and need a bed which gives good back support,
Our base type is a sprung slatted base
Can I have some recommendations on some of your mattress please?